1. Field of the Invention
The present invention relates generally to methods and apparatus for nucleating the crystallization of supercooled materials such as those used for phase-change thermal energy storage (e.g., salt hydrates such as sodium acetate, sodium thiosulfate, and trimethylol ethane hydrate). Such nucleation forces the supercooled material to change from its high-temperature phase to its low-temperature phase very rapidly and, in doing so, surrender its latent heat of transition. The invention relates more specifically to a method and apparatus for controlling and triggering nucleation at specific temperatures and times through manual, thermal, or electromechanical triggering.
2. Description of Prior Art
The general principles of thermal energy storage by means of phase-change materials are widely known. As materials are heated from a beginning state or phase, the solid phase for example, to the liquid phase, energy is absorbed. In the temperature range at which the material changes from one phase to another, more energy is required to raise the temperature an additional increment than to raise the temperature by the same increment when the material is not changing phase. This additional energy required at the phase change (or transition from one state to another) of the material is called the latent heat of transition.
More specifically, the heat required for the phase change from liquid to gas and is called latent heat of vaporization. The heat required for the phase change from solid to liquid and given up in the reverse phase change from liquid to solid is known as the latent heat of fusion. When a material cools, the energy absorbed at the phase-change point is normally given up. Some materials will cool well below the normal phase change temperature, but still retain the latent heat of transition and remain in the higher-temperature phase or state. For example, some materials under some circumstances may be cooled below the temperatures at which they normally change from liquid to crystalline, yet remain in the liquid state, thus still retain the latent heat of fusion. A material in this condition is said to be undercooled or supercooled. It is possible to create conditions in an undercooled material that will cause it to change very rapidly from the high-temperature phase to the low-temperature phase, thus giving up the energy stored as the latent heat of transition or fusion rapidly. The energy so surrendered may potentially be put to practical use in many ways.
A major drawback to effective use of the latent heat of transition or fusion of supercooled thermal storage materials has been the inability to control the conditions of release of the energy so that it can be accomplished reliably and predictably when desired by the user. There have been previous devices and methods developed for inducing the crystallization of undercooled or supercooled materials. For example, U.S. Patent No. 4,077,390 issued to J. Stanley, et al., utilizes a method of bending a ferrous metal strip positioned in a supercooled sodium acetate solution. Stanley et al, teach that crystallization of the supercooled solution may be caused by the flexing of the ferrous metal strip. Somewhat similarly, U.S. Patent No. 2,220,777, issued to D. Othmer, teaches the scratching or rubbing together of two pieces of metal or other materials in a sodium acetate solution in order to begin the crystallization of the solution.
U.S. Patent No. 1,384,747, issued to Eckelmann et al., crystallizes a sodium acetate solution by disturbing it with a rod or by exposing the solution to air. Likewise, U.S. Patent No. 1,385,074, issued to G. Ferguson, teaches a method of crystallizing a sodium acetate solution by piercing a seal to expose the solution to air and, then shaking the solution; and U.S. Patent No. 1,920,853, issued to R. Ferguson, also exposes sodium acetate solution to air in a valve mechanism to begin crystallization of the solution.
U.S. Patent No. 4,512,846, issued to Schlicta, teaches placing a solution of material to be crystallized in a pressure vessel and subjecting it to extremely high pressure to keep the solution in a supercritical state and then providing a seed crystal so that slow growth of a large crystal may proceed on the seed crystal.
Russian Patent No. 488,611, issued to Kurtzman, discloses apparatus that permits a crystal grower to drop a seed crystal into a supersaturated solution in order to initiate crystal growth. Besides application such as those described in the earlier patents above, where the undercooling of a material is a desired way of storing and then releasing on demand the latent heat of transition or fusion, there are also circumstances in which undercooling is an undesired effect. For example, in a process where crystallization of a material is desired to occur at a predictable temperature or at a controlled rate, the undercooling phenomenon can result in the material remaining in the liquid phase, even through it has been cooled to below its normal freezing or crystallization temperature, thus impeding or blocking the desired crystallization process. In such circumstances, an automatic, reliable means of preventing such undercooled condition would be very beneficial, but has not been available prior to this invention.
While all of the above-described prior developments were effective to various extents for particular purposes, there has remained a fundamental inability to trigger nucleation of the crystallization process at precise, predetermined times or temperatures with automatic, or at least easily controlled, trigger mechanisms.